The hematopoietic stem cell (HSC), through proliferation and differentiation, gives rise to all lymphoid, myeloid, and erythroid cells. Pluripotent HSCs are thus the basis of bone marrow transplantation and are considered attractive target cells for hematopoietic gene therapy for many clinical conditions. However, these important clinical applications have been severely hampered by the low numbers of HSCs that can be obtained from an animal, as well as difficulties in culturing HSCs in vitro and expanding HSCs for subsequent administration to a patient.
Culture and propagation of stem cells, such as hematopoietic stem cells, typically requires supplementation with unknown factors that allow the stem cells to survive and multiply in number. The unknown factors can be supplied by co-culturing the stem cells with feeder cells which secrete an undefined panel of factors, or can be supplied by adding undefined serum products to the growth medium. Such supplemented medium contains many unknown factors and therefore is not chemically defined.
The presence of unknown factors is problematic when the stem cells are being prepared for in vivo use, especially in humans. In many instances, the unknown factors are from non-human sources (such as bovine serum products). The non-human components may cause an immune reaction in the recipient, or the undefined components could include undetected pathogenic agents such as prions or viruses that would be detrimental to the recipient of the stem cells. There is a need for methods and compositions that allow the in vitro and/or ex vivo propagation of human HSCs in a chemically defined medium, while maintaining the pluripotency of the propagated cells.